Radio-frequency attenuator

ABSTRACT

A simple single-turn attenuator device having a single rotor carrying three interconnected resistive elements on a hightemperature resistant ceramic rotor plate and positioned for simultaneous arcuate movement past respective wiper-terminal members fixed in the attenuator housing. The attenuator is adapted to replace ganged-potentiometer devices; and may include on the rotor a fixed-value resistor element in series with one of the resistive elements.

United States Patent n 1 Bruder [451 July 31, 1973' 1 RADIO-FREQUENCY ATTENUATOR [75] Inventor:

Frank J. Bruder, Newport Beach, Calif.

[73] Assignee: Bourns, 1nc., Riverside, Calif.

[22] Filed: Jan, 10, 1972 [21] Appl. N0.: 216,683

[52 US. Cl 338/150, 338/137, 338/142 [51] Int. Cl 1101c 5/00 [58] Field of Search 338/120, 122, 123, 338/128,125,127,184,199,137, 68,150, 142; 333/81 R [56] References Cited UNITED STATES PATENTS 3,602,867 8/1971 Kohler 338/120 3,657,689 4/1972 Casey et a1. 388/128 X 3,564,476 2/1971 Barden 338/128 X 2,873,336 Tassara 338/127 5/1961 Deniston 338/125 7/1959 Burden 338/184 X OTHER PUBLICATIONS G.W.A. Dummer, Fixed Resistor, pagel27, 1967, Sir IsaacPitman & Sons Ltd., London.

Primary Examiner-Richard B. Wilkinson Assistant Examiner-N. Weldon s7 ABSTRACT A simplesingle-tum attenuator device having a single rotor carrying three interconnected resistive elements on a high-temperature resistant ceramic rotor plate and positioned for simultaneous arcuate movement past respective wiper-terminal members fixed in the attenuator housing. The attenuator is adapted to replace ganged-potentiometer devices; and may include on the rotor a fixed-value resistor element in series with one of the resistive elements,

7 Claims, 7 Drawing Figures I PATENIED JUL 3 1 I975 PRIOR ART 1 RADIO-FREQUENCY ATTENUATOR BRIEF SUMMARY OF THE INVENTION" shaft or by interconnection of the shafts of the individv ual units. The terminals were externally so wired as to provide, for example, a bridged-T network, with additional fixed resistors, if necessary, externally bridging I appropriate terminals. One disadvantage of the noted prior-art attenuators is that the, structure is large and comprises a large number of parts'including three housings, three rotors, andthree sets of terminals. Another disadvantage is that while in the interest of economy the three ganged potentiometers thus concatenated are of similar electrical construction, only that unit comprising the mounting bu shing and located in contact with the panel on whichthe attenuator is mounted has opportunity forv good heat dissipation. The consequence is that the units most remote from the panel require electrical de-rating; and thus none of the'units can logically be operated at apower-rating more than a minor fraction of the design power-rating at which it could be safely operated as a single potentiometer. Eurther, the prior art attenuator devices, due to interconnecting leads and the like'which insert'between the active resistances undesirable reactances, are'restricted to relatively low radio-frequency spectra. For example, the prior art devices'suffere'd serious degradation of signals in frequency ranges aboveabout 12 5.ml-I z.'

b. The present invention. The aforenoted and other disadvantages including large manufacturing costs, are eliminated or greatly reduced by the inventionherein described; and the I practical operating frequency ranges that may be accommodated without serious degradation are increased to include signals of frequency in excess of 500 ml-lz; The invention comprehends a set or plurality of resistive elements all disposed on a face of a single insulative disc-like rotor member mounted for arcuate rotational movement about an axis, in a single housing the cover member of which carries three integral wiper-terminal units disposed. to present three terminals outsidev the housingfor-external electrical connections and disposed to present,iinside the housing, three wiper or contact means in resilient contacting engagement with respective ones ofthe plurality of resistive elements. The disc-like rotor member is constructed to provide excellent conduction of heat away from the resistive elements; and the latter, being all on the same member and having substantially equal opportunity for heat-transfer, need not suffer electrical de-rating. The rotor member maybe, for example of alumina or like insulative material characterized by running in a metal bushing forming a part of theihousing and providing heat-conductance and heat-sink ing an integral bushing, an alumina rotor disc is swagemounted on the inner end of .the shaft with its resistive cermet resistance-element array facing the contact points of a set of contact-terminal members that are molded in place in a housing cover that is bayonetlocked to the housing body. The bushing may be secured in anaperture of a panel, as by conventional nut means that are not of the present invention.

A principal object of the invention is to provide gen- 'eral improvements in variable plural-element attenuator means. An additional object is to provide a simple inexpensive variable attenuator structure. Another object of the invention is to provide an attenuator structrue comprising a minimum of components. Another object of the invention is to provide improvements in heat-dissipation in multi-element variable'rotary attenuators. Another and important object is to provide a r. f. attenuator capable of effective operation in fre-, quency ranges including 500 mI-lz. Other objects and advantages of the invention are hereinafter set out or :made evident in the appended claims and following detailed description of a preferred embodiment of the invention as illustrated inthe accompanying drawings.

' DESCRIPTION OF THE DRAWINGS In the drawings, I FIG. 1V is a pictorial view of arepresentative. specimen of prior-art plural-element rotaryvariable attenuator;

FIG. 2 is a schematic diagram illustrating connections for'the electrical components of the attenuator structure depicted'in FIG. 1, to form a variable T- attenuator.

3 'FlG..2a is anelectrical schematic diagram of a typical attenuator circuit commonly used in a variety of electronic apparatuses and as commonly shown in elementary circuit diagrams;

shown in FIG. 3, to a larger scale, illustrating relative "placement of the parts of the preferred embodiment, in

operative juxtaposition.

DETAILED DESCRIPTION As willbe noted from FIGS. 1 and 2, the conventionalT-pad'type of radio-frequency attenuator 10 paths for the disc. The entire physical structure, be-

yond a cermet array on a ceramic'disc, comprises essentially only a rotor shaft or rotator, two housing members, and three terminal-wiper'members, all of which are simple and inexpensive to produce and assemble. In a preferred form herein described in detail a metal shaft is molded in'place in a housing body havcompriseda set of three potentiometers l2, l4 and 16 disposed in ganged relationship with a single actuating shaft 18 operative to synchronously rotate the contactcarrying wipers 12w, 14w, and 16w, as indicated by the interconnecting dash-line linkage in FIG. 2. Commonly a fixed-value resistor Rs is connected in the circuitry as indicated. As is known, the basic purpose isto permit a change of signal amplitude or potential into a following circuit from a preceding circuit without significantly changing thefimpedance as viewed from either in the direction of the other. That'function is accomplished by varying the amount of the output power of the signal fromthe preceding circuit that is dissipated in resistance'means shunted acrossrand in series in the signal line connecting the two circuits, from zero value at one extreme to a very large percentage at the other extreme. The technology involved in, or related to, design and use of such attenuators is well understood in the radio-frequency signaling art and is set forth in standard texts, and so is not further related in detail herein.

The present invention grossly reduces the number and costs of parts of a variable attenuator of the noted class, concurrently therewith greatly improves heat dissipation from the resistive elements of the attenuator, and further greatly reduces the space required by the instrument. Further the maximum frequency-range over which satisfactory operation is feasible is substantially doubled. As depicted in preferred exemplary form in FIGS. 3 6, the exemplary attenuator, denoted generally by the number 20, comprises as principal physical components a housing body 22, a housing cover 24, a rotor shaft 26 and a rotor disc 28. The housing body 22 comprises a cup-shaped open-end receptacle portion 22a (FIG. 4) and integral therewith a threaded bushing portion 22b, both having an axial bore 220 (FIG. 6) therethrough and both concurrently formed of injection-molded glass-filled thermoplastic resin. The body 22 is provided, adjacent the inside of the rim portion, with key-receiving recesses such as that at 22n (FIG. 4), for reception of respective complementary keys such as that at 24n on cover 24. The cover is thus adapted to be secured to the housing body by a twist-lock action and means, and sealed against ingress of undesirable material by adhesive applied on the edge of the outer rim of the body at the time the cover is assembled onto the body; I

The shaft 26, by means of which the attenuatoris made adjustable by manual manipulation, is, in the preferred form shown in FIG. 6, mounted in the body 22 when the latter is molded. That is, the shaft is molded in and due to the shaped shank including a section of reduced diameter and with shoulders as shown, is secured in place without retaining-clip or like means. At its outer end the shaft is adapted to receive torque applied by a tool or other means, for example, a screwdriver. However, as will be evident to those skilled in the rotary potentiometer art, the shaft may repose in a bore of uniform diameter and be fitted with C-ring retainer means to prevent axial displacement, or may have a flange at its inner end and retainer-clip means at its outer end. At its inner end the rotor shaft 26 is formed with a disc-supporting formation for secure attachment thereto of the rotor disc 28, the formation as shown including a rearwardly facing annular shoulder 22s (FIG. 6) and a stub end 22c of the shaft protruding beyond the shoulder and headed or peened over the outer face 28f of the rotor disc. The larger-diameter portion of the shaft, as shown, extends axially into the recess or cavity formed by the housing base; and shoulder 22s is so spaced that the rotor disc 28 is rotatable free of contact with the housing, and with minimum feasible axial freedom.

The rotor disc 28 (FIG. is formed of heatconductive insulation such as alumina ceramic, as a circular wafer with a central shaft-receiving aperture 28h for snug reception of stub end 22e of the shaft 26. On its exposed face the rotor disc bears a plurality of resistive tracks, preferably of cermet material fired onto the disc, and each formed of shape and material and thickness such as to vary non-linearly according to a respective particular resistance-versus-displacement equation or graphical representation as required for a designed circuit. The resistive tracks are interconnected on the disc in accord with the attenuator circuit required. Exemplary interconnected resistive tracks, three in number and arranged for a T-pad network, are illustrated in FIG. 5, together with a connected resistor equivalent to resistor Rs of the circuit indicated in FIGS. 2 and 2a. In the exemplary resistive network illustrated in FIG. 5, the fixed-value resistor Rs, equivalent to resistor Rs in the circuit of FIG. 2, extends through an angular arc of approximately and the three exemplary resistive tracks R1, R2 and R3, corresponding toRl, R2 and R3, respectively, of FIG. 2a, extend through respective arcs as shown, within the remainder of approximately 270 not occupied by the fixed-value resistor Rs. The resistance-tapers or changes of resistance value relative to angular displacement from an end of a track such as at A', are produced by application of different layers of cermet materials of differing width and/or thickness, depending upon the resistance vs. contactdisplacement curve or function desired, in accord with principles followed or practiced in the known art of non-linear element film-type potentiometers. Examples of modes of producing single-element non-linear potentiometers are shown in U.S. Pat. Nos. 432,279; 2,061,107; and 2,082,980. As an example, that part of the resistive track R2" connecting with Rs. at the top of FIG. 5 may be of 17.3 ohm-cm. resistivity, blending with a tapered central portion of 20.3 ohm-cm. resistivity, blending with a tapered narrower lower end portion of 70.8 ohm-cm. resistivity, the taper decreasing to a point of nearly infinite resistance value at the pointed end of the track. Similarly, the tracks R1 and R3 are produced to have desired resistance vs. displacement variations, by way of use of cermet materials of various resistivities along the track, and/or plural layers of ma-' terials of the same or differing resistivities, different tapers, etc.

The shaft 26 may be made of aluminum alloy or other material of high heat-conductivity. Thus while all of'the attenuator resistive elements and resistors are on a single small substrate, they are of high operatingtemperature rating and are in direct contact with a highly heat-conductive substrate which has, due to the close-fitting and large area connection with shaft 26, an excellent heat-transfer capability. Thus the three exemplary resistive elements, despite being in close adjacency each to another, are characterized by high operating capability. Since the interconnections between the resistive members are of zero length, interconnecting lead reactance is reduced to an absolute minimum in contrast to that presented by the soldered wires interconnecting terminals as shown in FIG. 1.

The cover 24 is molded with three integral terminalcontact members or devices Tl-Cl, T2-C2, and T3-C3, as inserts each having a respective contactmember or limb extending from the inner face of the cover, and each having a respective terminal extending from the periphery or outer surface of the cover. As illustrated in FIG. 4, each of the contact members or limbs may be produced to comprise several independently operable resilient contact fingers each with a contact point positioned to wipe or brush on the appropriate one of the resistive elements on the rotor disc 28. Thus contact Cl is positioned for engagement of its points on resistive element R3, while contacts C2 and C3 are positioned for electrical engagement with elements R1 and R2, respectively. The contact members are formed of resilient alloy such as nickel silver, and are so disposed relative to the inner face of cover 24 that when the cover is installed the contact limbs are stressed to provide a determined amount of contactpoint force or pressure. The general shape and configuration of the terminal-contact members is shown in FIG. 6, in which member T2-C2 is illustrated partly embedded in the cover 28.

It is evident that the illustrated structure comprising the exemplary embodiment of the invention is composed of but very few parts of inexpensive construction, that manufacture is greatly simplified over that involved in producing an attenuator of prior-art construction; and, more importantly, the device provides much better electricaland mechanical performance than the prior art devices, due to decrease of high frequency reactance as noted and absence of any mechanical coupling of movable brushes or wipers or rotors. All of the cermet resistive elements can be made on a single substrate and thus all the tracking problems incident to use of three physically separate rotors, etc. are eliminated or avoided. No interconnecting-lead reactance is present to degrade optimum electrical operation. Use of cermet resistive material on a ceramic substrate provides excellent electrical performance over a very wide frequency range, relative to the performance of, for example, carbon or metal film on an organic substrate, both of which latter materials exhibit nonlinear variation of electrical characteristics with change of frequency. While in the exemplary configuration shown and described, a T-pad attenuator has been chosen for illustration, it is evident that the invention equally comprehends H, L, pi, and like attenuator configurations. Thus the aforementioned objects of the invention are noted as being fully attained by the disclosed attenuator.

Having described a preferred embodiment of my invention in detail, I claim:

1. A radio-frequency attenuator comprising:

first means, including a housing providing an interior chamber and a bore extending into the chamber from the ambient exterior; second means, including a plurality of separate contact members and a terminal for each thereof, supported by said housing in electrically insulative arrangement with each other with the contact members disposed in said chamber; third means, including a rotor having a shaft rotatable in said bore and having an insulativerotor body having an exposed surface in said chamber and rotatable by said shaft;

and a fourth means, including at least three arcuate non-linear, generally tapered interconnected resistance element tracks on said rotor surface each disposed for rotary brushing interengagement with a respective one of said contact members, one end of each of at least three of said resistance tracks all being electrically connected together at a region on said rotor surface removed from the regions of rotary brushing interengagement of each of said contact members;

wherein said contacts are positioned relative to their respective resistive element tracks and said resistive element tracks are each formed of a shape, material and thickness such that the output sensed at each of said terminals varies in a predetermined relationship with the rotational displacement of said rotor;

whereby said attenuator comprises a minimum number of parts and provides superior electrical performance.

2. A radio-frequency attenuator as definedin claim 1, in which said housing comprises a cup-like body member with an integral bored bushing, and a cover member in which a plurality of integral terminalcontact devices are embedded each with a contact limb extending into said chamber and a terminal extending to the exterior of the housing,

whereby said attenuatoris inexpensive in manufacturing cost.

3. A radio-frequency attenuator according to claim 1, in which said fourth means comprises first, second and third arcuate resistive tracks on'the surface of said rotor body, and a connecting resistor thereon interconnecting an end of each of said first and third resistive tracks with an end of said second resistive track,

whereby there is provided a T-network of resistive elements without necessity for interconnecting leads and characterized by a minimum of high frequency reactance between the resistive elements.

4. Aradio-frequency attenuator as defined in claim 1, in which said shaft has a central portion of reduced diameter, and in which said housing comprises a body member and a cover member, said body member being of molded construction and said shaft being an insert with said central portion permanently embedded therein, 1

whereby necessity for shaft-retaining means is obviated and the cost of said attenuator is minimized.

5. An attenuator as in claim 1 wherein the other end of each of said joined three resistance tracks is electrically insulated from any portion of any of said joined three resistance tracks.

6. An attenuator as in claim 1 wherein said resistance tracks are connected together and positioned in a T-pad attenuator configuration. U

7. A radio-frequency attenuator comprising:

first means, including a housing providing an interior chamber and a bore extending into the chamber from the ambient exterior;

second means, including a rotor having a shaft rotatable in said bore and having an insulative rotor body having an exposed surface in said chamber and rotatable by said shaft;

third means, including at least three arcuate resistance elementtracks on said rotor surface one end of each all being connected together and the other end of each being separated from said connected resistance tracks for providing a plurality of variable resistance portions and contact means for simultaneously, separately varying the resistance sensed along each track in accordance'with a respective, separate, predetermined relationship to angular rotation of said rotor, said contact means including a plurality of separate contact members electrically insulated from each other, one for each resistance element track, and a plurality of terminals, one electrically connected to each "of said contact members wherein said contact means is supported by said housing with said contact'members disposed in predetermined positions in said chamber in rotary brushing interengagement with their respective resistance element tracks; whereby said attenuator comprises a minimum number of parts and provides superior electrical performance.

t I i t 

1. A radio-frequency attenuator comprising: first means, including a housing providing an interior chamber and a bore extending into the chamber from the ambient exterior; second means, including a plurality of separate contact members and a terminal for each thereof, supported by said housing in electrically insulative arrangement with each other with the contact members disposed in said chamber; third means, including a rotor having a shaft rotatable in said bore and having an insulative rotor body having an exposed surface in said chamber and rotatable by said shaft; and a fourth means, including at least three arcuate non-linear, generally tapered interconnected resistance element tracks on said rotor surface each disposed for rotary brushing interengagement with a respective one of said contact members, one end of each of at least three of said resistance tracks all being electrically connected together at a region on said rotor surface removed from the regions of rotary brushing interengagement of each of said contact members; wherein said contacts are positioned relative to their respective resistive element tracks and said resistive element tracks are each formed of a shape, material and thickness such that the output sensed at each of said terminals varies in a predetermined relationship with the rotational displacement of said rotor; whereby said attenuator comprises a minimum number of parts and provides superior electrical performance.
 2. A radio-frequency attenuator as defined in claim 1, in which said housing comprises a cup-like body member with an integral bored bushing, and a cover member in which a plurality of integral terminal-contact devices are embedded each with a contact limb extending into said chamber and a terminal extending to the exterior of the housing, whereby said attenuator is inexpensive in manufacturing cost.
 3. A radio-frequency attenuator according to claim 1, in which said fourth means comprises first, second and third arcuate resistive tracks on the surface of said rotor body, and a connecting resistor thereon interconnecting an end of each of said first and third resistive tracks with an end of said second resistive track, whereby there is provided a T-network of resistive elements without necessity for interconnecting leads and characterized by a minimum of high frequency reactance between the resistive elements.
 4. A radio-frequency attenuator as defined in claim 1, in which said shaft has a central portion of reduced diameter, and in which said housing comprises a body member and a cover member, said body Member being of molded construction and said shaft being an insert with said central portion permanently embedded therein, whereby necessity for shaft-retaining means is obviated and the cost of said attenuator is minimized.
 5. An attenuator as in claim 1 wherein the other end of each of said joined three resistance tracks is electrically insulated from any portion of any of said joined three resistance tracks.
 6. An attenuator as in claim 1 wherein said resistance tracks are connected together and positioned in a T-pad attenuator configuration.
 7. A radio-frequency attenuator comprising: first means, including a housing providing an interior chamber and a bore extending into the chamber from the ambient exterior; second means, including a rotor having a shaft rotatable in said bore and having an insulative rotor body having an exposed surface in said chamber and rotatable by said shaft; third means, including at least three arcuate resistance element tracks on said rotor surface one end of each all being connected together and the other end of each being separated from said connected resistance tracks for providing a plurality of variable resistance portions and contact means for simultaneously, separately varying the resistance sensed along each track in accordance with a respective, separate, predetermined relationship to angular rotation of said rotor, said contact means including a plurality of separate contact members electrically insulated from each other, one for each resistance element track, and a plurality of terminals, one electrically connected to each of said contact members wherein said contact means is supported by said housing with said contact members disposed in predetermined positions in said chamber in rotary brushing interengagement with their respective resistance element tracks; whereby said attenuator comprises a minimum number of parts and provides superior electrical performance. 